Oriented polyester film, particularly biaxially oriented film composed of polyethylene terephthalate (PET), is widely used as a base for drafting film, photographic film and reprographic film, as well as for packaging and labelling applications.
Because PET film is hydrophobic and is not readily receptive to coating, in most applications where the film is to serve as a base or support for other coatings, it must be first coated on one or both sides with a primer coating which adheres to the film and is receptive as well to other coatings applied to it. For example, U.S. Pat. Nos. 2,627,088 and 2,698,240 teach a primer coating for PET film comprising a terpolymer composition of vinylidene chloride, acrylic ester and itaconic acid. This primer layer is said to have excellent adhesion to the polyester surface and to water or alcohol based photographic gelatin layers subsequently coated thereon.
Another known primer is the thermoset acrylic or methacrylic coatings taught in U.S. Pat. No. 3,819,773, which can be applied to the PET film in the plant from aqueous medium. Such a primer layer enhances the adhesion of organic solvent based reprographic and drafting layers applied thereto.
British Patent No. B 1,127,076 teaches the application to PET film of a primer layer which may comprise an aqueous latex of non-crosslinked polyvinyl acetate. The patent indicates that such primer layers render the film heat-sealable. British Patent No. 1,146,215 teaches the application to PET film of a primer coating comprising an aqueous dispersion of a copolymer of vinyl acetate (100 parts), alkyl methacrylate (10 parts) and itaconic acid (6 parts). The patent indicates that the primer provides enhanced adhesion to aqueous based gelatin photographic emulsions, but does not indicate that organic solvent based coatings may be successfully applied thereto. U.S. Pat. No. 3,674,531 discloses the application to PET film of a primer layer comprising a copolymer of a vinylhalogenoester, such as vinyl chloroacetate, which may be copolymerized with numerous different monomers such as acrylic and methacrylic acids, esters and amides, olefins and vinyl alcohol. This copolymer may also be crosslinked such as by the inclusion of a melamine or urea formaldehyde crosslinking agent. The primed PET film is stated to exhibit enhanced adhesion to a variety of coatings applied thereto, including reprographic coatings. U.S. Pat. No. 3,624,021 discloses a matte coating composition for PET film based on a mixture of a vinyl acetate/crotonic acid copolymer and an excess of a urea formaldehyde resin which presumably crosslinks upon the application of heat. British Patent 1,497,657 discloses a similar primer coating for PET film which is based on a mixture of an unhydrolysed polymer or copolymer of vinyl acetate and one or more synthetic resins capable of intra-molecular crosslinking, such as a melamine formaldehyde resin. The patent indicates that the primer layer may serve, inter alia, as an anchor layer for the application of further coatings, such as light-sensitive layers. A similar disclosure is found in U.S. Pat. No. 4,066,820. Other patents disclosing the application of crosslinkable vinyl acetate polymers to various surfaces are found in U.S. Pat. No. 2,553,524 and U.S. Pat. No. 3,307,690.
U.S. Pat. No. 3,720,539 discloses a primer coating system for polyester film comprising a first undercoat based on a composition containing a polyacrylamide or polyvinyl alcohol polymer and having applied over such undercoat a coating comprising a mixture of polyvinyl acetate and a melamine/formaldehyde resin. Although this system adheres well to the film, it is a complicated formulation involving two coating steps and also includes materials such as trichloroacetic acid which are not especially desirable in such coating formulations.
British Patent No. 1,168,171 teaches the application to PET film of a primer layer described as a curable copolymer of an aliphatic unsaturated aliphatic acid and at least one other ethylenically unsaturated monomer. The patent indicates that such a primer coating enhances adhesion to aqueous based gelatin photographic emulsions, but does not indicate that organic solvent based coatings may be successfully applied thereto.
While some of these and other polyester film primer layers are effective in enhancing the adhesive qualities of PET film, it is important for the film manufacturer that scrap film made during production must be recyclable through the film-forming process. Scrap film is normally comminuted, melted, extruded into the form of pellets, mixed with fresh polyester, re-melted and re-fed to the film-forming extruder. Temperatures of about 270.degree. C. to 310.degree. C. may be encountered during such processing of PET reclaim film. Many of the primer compositions discussed above are not stable at such temperatures and tend to impart an undesirable yellow or black discoloration to finished oriented PET film containing significant amounts of such reclaim film, particularly after repeated passes through the extruder. Such is the case with the vinylidene chloride-containing polymers used as PET primer layers and disclosed in U.S. Pat. Nos. 2,627,088 and 2,698,240. It is also the case with primer layers based on copolymers containing vinyl chloroacetate as disclosed in U.S. Pat. No. 3,674,531, as well as primer layers based on a mixture of a vinyl acetate polymer or copolymer and a separately added resinous crosslinking agent such as disclosed in a number of the patents referred to above, or crosslinking agents such as Lewis acids or Werner complexes. It has been found that discoloration and degradation of these primer layers during the reclaim process is most likely attributable to the evolution of chlorine gas or hydrogen chloride in the case of chlorine-containing primer layers, and the relative heat instability of condensation type resinous crosslinking agents such as melamine or urea formaldehyde resins.
Thus, whereas a wide variety of primer coating compositions for PET film are known in the art, the number of such coatings available to the film manufacturer, who requires not only enhanced adhesion to PET film of solvent based coatings applied thereto, but also a primed film which is commercially reclaimable, are limited.
Accordingly, it is an object of this invention to provide an oriented self-supporting polyester film material receptive to organic solvent based coatings.
Another object of this invention is to provide a primer coating for oriented polyester film material which can be safely and easily applied in the plant during production of the film, and to provide a primed film which can be successfully reclaimed in the plant.